1. Field of the Invention
This invention relates to a torque sensor, particularly to a contactless (non-contact) magnetostrictive torque sensor.
2. Description of the Related Art
The contactless magnetostrictive torque sensor generally comprises a magnetic metal film exhibiting uniaxial magnetic anisotropy that is attached to a torque transmission shaft and an exciting coil and a detector coil each installed, at a location near the magnetostrictive film. Changes in inductance owing to permeability fluctuation produced in the magnetostrictive film by the applied torque are detected by the detector coil as potential differences from which the magnitude of the applied torque is detected.
A contactless magnetostrictive torque sensor of this type is taught, for instance, by Japanese Laid-Open Patent Application 2001-133337 (e.g., paragraph numbers 0036 to 0041, FIGS. 5 and 6, and elsewhere). This conventional torque sensor is used to detect steering torque produced in a vehicle electric power steering system that uses an electric motor to provide steering torque assistance.
A basic requirement of a torque sensor is that it should be able to accurately detect the magnitude and direction of torque applied to the torque transmission shaft. However, when a torque sensor is installed in an electric power steering system, as in the case of the prior art reference cited above, a particular problem arises owing to the fairly large amount of current passed through the electric motor. Specifically, noise produced by current passing through the motor is liable to degrade the detection accuracy when the level of the torque sensor detection current (voltage) is low. Many types of electrical equipment other than the electric power steering system also produce noise that can affect detection accuracy.
Further, when a torque sensor is used in the electric power steering system of a vehicle, for example, it is liable to become incapable of reliable detection owing to changes in its detection characteristics caused by extreme temperature variation in the vehicle running environment depending on the region and season, and because of exposure to heat generated by the motor or vehicle engine.